Method of operation and regulation of a vapour compression system

ABSTRACT

The present invention involves a compression refrigeration system including a compressor, a heat rejector, expansion means and a heat absorber connected in a closed circulation circuit that may operate with supercritical high-side pressure. An apparatus and method are provided to optimize energy efficiency.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a compression refrigeration systemincluding a compressor, a heat rejector, an expansion means and a heatabsorber connected in a closed circulation circuit that may operate withsupercritical high-side pressure, using carbon dioxide or a mixturecontaining carbon dioxide as the refrigerant in the system.

2. Description of Related Art

Conventional vapour compression systems reject heat by condensation ofthe refrigerant at subcritical pressure given by the saturation pressureat the given temperature. When using a refrigerant with low criticaltemperature, for instance CO₂, the pressure at heat rejection will besupercritical if the temperature of the heat sink is high, for instancehigher than the critical temperature of the refrigerant, in order toobtain efficient operation of the system. The cycle of operation willthen be transcritical, for instance as described in WO 90/07683.Temperature and high-side pressure will be independent variables,contrary to conventional systems.

WO 94/14016 and WO 97/27437 both describe a simple circuit for realisingsuch a system, comprising a compressor, a heat rejector, an expansionmeans and an evaporator connected in a closed circuit. CO₂ is thepreferred refrigerant for both systems.

The system coefficient of performance (COP) for transcritical vapourcompression systems is strongly affected by the level of the high sidepressure. This is thoroughly explained by Pettersen & Skaugen (1994),which also presents a mathematical expression for the optimum pressure.Because high side pressure is not a function of temperature, high sidepressure can be controlled in order to achieve optimum energyefficiency. To do so it is necessary to determine optimum pressure forgiven operating conditions.

Several publications and patents are published which suggest differentstrategies to determine the optimum high side pressure. Inokuty (1922)published a graphic method already in 1922, but it is not applicable forthe present digital controllers.

EP 0 604 417 B1 describe how different signals can be used as steeringparameter for the high side pressure. A suitable signal is the heatrejector refrigerant outlet temperature. The correlation between optimumhigh side pressure and the signal temperature is calculated in advanceor measured. Densopatent describes more or less an analogous strategy.Different signals are used as input parameters to a controller, whichbased on the signals regulates the pressure to a predetermined level.

Among others, Liao & Jakobsen (1998) presented an equation whichcalculates optimum pressure from theoretical input. The equation doesnot take into account practical aspects which may affect the optimumpressure significantly.

Most methods for optimum pressure determination described above take atheoretical approach. This means that they are not able to compensatefor practical aspects like varying operating conditions, and theinfluence of oil in the system. Optimum pressure is thus frequentlydifferent from the calculated one. There is also a risk for a “wind up”and lack of control. This happens when a temperature signal gives afeedback to the controller, which adjust the pressure with some delay.If conditions change rapidly, the controller will never establish aconstant pressure, and cooling capacity will vary.

As explained above, it is a possibility to run tests and measure optimumhigh side pressure relations. But this is time consuming and expensive.Furthermore, it is hard, if not impossible, to cover all operatingconditions, and the measurements have to be performed for all newdesigns.

BRIEF SUMMARY OF THE INVENTION

A major object of the present invention is to make a simple, efficientsystem that avoids the aforementioned shortcomings and disadvantages.

The invention is characterized by the features as defined in theaccompanying claims. Advantageous features of the invention are alsodefined therein.

The present invention is a new and novel method for optimum operation ofa system with respect to energy efficiency, the system comprising atleast a compressor, heat rejector, expansion means, and a heat absorber.

When operating conditions change, the controller in the transcriticalvapour compression system can perform a perturbation of the high sidepressure and thereby establish a correlation between the pressure andthe energy efficiency, or a suitable parameter reflecting the energyefficiency. A correlation between high side pressure and energyefficiency can then easily be mapped, and optimum pressure determinedand used until operating conditions change. This is a method which willwork for all designs of transcritical vapour compression systems. Noinitial measurements have to be made, and practical aspects will beaccounted for on site.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described in the following by way ofexamples only and with reference to the drawings in which,

FIG. 1 illustrates a simple circuit for a vapour compression system.

FIG. 2 shows a temperature entropy diagram for carbon dioxide with anexample of a typical trans-critical cycle.

FIG. 3 shows a schematic diagram showing the principle of optimum highside pressure determination. Temperature approach is used as COPreflecting parameter in the figure.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a conventional vapour compression system comprising acompressor 1, a heat rejector 2, an expansion means 3 and a heatabsorber 4 connected in a closed circulation system.

FIG. 2 shows a transcritical CO₂ cycle in a temperature entropy diagram.The compression process is indicated as isentropic from state a to b.The refrigerant exit temperature out of the heat rejector c is regardedas constant. Specific work, specific cooling capacity and coefficient ofperformance are explained in the figure.

As mentioned above, there is a mathematical expression for high optimumhigh side pressure in a trans-critical vapour compression system. Theexpression is as follows:

$\left( \frac{\partial h_{c}}{\partial p} \right)_{T} = {- {ɛ\left( \frac{\partial h_{b}}{\partial p} \right)}_{s}}$

The optimum pressure is achieved when the marginal increase of capacity(change of h_(c) at constant temperature) equals ε times the marginalincrease in work (change of h_(b) at constant entropy).

Perturbation of the high side pressure, is in principle a practicalapproach to use the equation above. By mapping the energy efficiency, ora parameter which reflects the energy efficiency, as function of highside pressure, it is possible to establish the point where the marginalincrease of capacity equals ε times the marginal increase in work.

Various parameters can be used as reflection for the energy efficiency.

EXAMPLE 1

The temperature difference between refrigerant and heat sink at the coldend of the heat rejector 4, is often denoted as “temperature approach”for a transcritical cycle. There is a correlation between high sidepressure and the temperature approach. An increase of the high sidepressure will lead to a reduction of temperature approach. The high sidepressure can favourably be increased until a further increase does notlead to a significant reduction of temperature approach. At this point,optimum high side pressure is established, and the system can beoperated at optimum conditions, maximizing the system COP. Thisprinciple is illustrated in FIG. 3.

A perturbation of the high side pressure will produce a relation asindicated in FIG. 3. When operating conditions change, or for otherreasons, a new perturbation can be made and a new updated relationestablished. In this way, the transcritical system will always be ableto operate close to optimum conditions.

EXAMPLE 2

Instead of using the temperature approach, it is an option to use thegas cooler outlet temperature as parameter for reflection of energyefficiency.

EXAMPLE 3

By measurements of system pressures and temperatures, it is possible toautomatically calculate the enthalpies for a transcritical cycle at thepoints 1 to 4 indicated in FIG. 2, if the refrigerant properties areknown. The enthalpies can be used for calculation of the systemcoefficient of performance. A perturbation of the high side pressurewill then produce a relation between COP and the high side pressuredirectly.

If COP is used as steering parameter, the optimum high side pressurewill be established directly. If a COP reflecting parameter is used, anexact measure for the “marginal effect” on the parameter has to bequantified. This measure can however easily be estimated. Anotherpossibility is to increase pressure until the parameter reaches apredetermined level.

1. A compression refrigeration system comprising: a closed circulationcircuit comprising a compressor, a heat rejector, an expansion device,and a heat absorber, said closed circulation circuit being operable tocirculate a refrigerant and pressurize the refrigerant to a high-sidepressure, the high-side pressure being supercritical; and a controlleroperable to estimate a parameter value reflecting energy consumption todetermine an optimum high-side pressure by perturbation of the high-sidepressure during operation of said compression refrigeration system;wherein said compression refrigeration system operates at the optimumhigh-side pressure after the optimum high-side pressure has beendetermined.
 2. The compression refrigeration system of claim 1, whereinsaid closed circulation circuit includes the refrigerant, and saidrefrigerant comprises carbon dioxide.
 3. The compression refrigerationsystem of claim 1, wherein the parameter value reflects minimum operableenergy consumption.
 4. The compression refrigeration system of claim 1,wherein said heat rejector lowers a temperature of the refrigerant, saidheat rejector utilizing a heat sink; and wherein the parameter value isa difference in temperature between the refrigerant and the heat sink.5. The compression refrigeration system of claim 1, wherein said heatrejector lowers a temperature of the refrigerant, said heat rejectorutilizing a heat sink; and wherein said controller estimates theparameter value by increasing the high-side pressure, monitoring animpact of increasing the high-side pressure on a difference intemperature between the refrigerant and the heat sink, and discontinuingincreasing the high-side pressure when the impact is below a thresholdlevel.
 6. The compression refrigeration system of claim 5, wherein thethreshold level varies according to at least one operating condition. 7.The compression refrigeration system of claim 1, wherein the parametervalue is an outlet temperature of said heat rejector.
 8. The compressionrefrigeration system of claim 1, wherein said controller estimates theparameter value by varying the high-side pressure and determining theoptimum high-side pressure corresponding to a minimum operable energyconsumption of the compression refrigeration system.
 9. The compressionrefrigeration system of claim 1, wherein said compressor pressurizes therefrigerant to the optimum high-side pressure after the optimumhigh-side pressure has been determined.
 10. The compressionrefrigeration system of claim 1, wherein said controller controls aperturbation of the high-side pressure and establishes a correlationbetween the high-side pressure and the parameter value, the parametervalue reflecting a minimum operable energy consumption.
 11. A method ofoperating a compression refrigeration system including a closedcirculation circuit comprising a compressor, a heat rejector, anexpansion device, and a heat absorber, the method comprising: operatingthe compression refrigeration system by circulating a refrigerantthrough the closed circulation circuit and pressurizing the refrigerantto a high-side pressure, the high-side pressure being supercritical;estimating a parameter value reflecting energy consumption to determinean optimum high-side pressure by perturbation of the high-side pressureduring operation of the compression refrigeration system; and operatingthe compression refrigeration system at the optimum high-side pressureafter the optimum high-side pressure has been determined.
 12. The methodof claim 11, wherein the refrigerant comprises carbon dioxide.
 13. Themethod of claim 11, wherein said estimating of the parameter valuecomprises: providing a controller which controls a perturbation of thehigh-side pressure and estimates the parameter value, the parametervalue reflecting minimum operable energy consumption.
 14. The method ofclaim 11, wherein said operating of the compression refrigeration systemcomprises the heat rejector lowering the temperature of the refrigerant,the heat rejector utilizing a heat sink; and wherein the parameter valueis a difference in temperature between the refrigerant and the heatsink.
 15. The method of claim 11, wherein said operating of thecompression refrigeration system comprises the heat rejector loweringthe temperature of the refrigerant, the heat rejector utilizing a heatsink; and wherein said estimating of the parameter value comprises:increasing the high-side pressure, monitoring an impact of increasingthe high-side pressure on a difference in temperature between therefrigerant and the heat sink, discontinuing increasing the high-sidepressure when the impact is below a threshold level.
 16. The method ofclaim 15, wherein the threshold level varies according to at least oneoperating condition.
 17. The method of claim 11, wherein the parametervalue is an outlet temperature of the heat rejector.
 18. The method ofclaim 11, wherein said estimating of the parameter value comprises:varying the high-side pressure; determining a high-side pressurecorresponding to a minimum operable energy consumption of thecompression refrigeration system.
 19. The method of claim 11, whereinsaid operating of the compression refrigeration system after the optimumhigh-side pressure has been determined comprises pressurizing therefrigerant to the optimum high-side pressure.
 20. The method of claim11, wherein said estimating of the parameter value comprises: providinga controller which controls a perturbation of the high-side pressure andestablishes a correlation between high-side pressure and the parametervalue, the parameter value reflecting a minimum operable energyconsumption.